package org.apache.lucene.search.function;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import org.apache.lucene.index.IndexReader;
import org.apache.lucene.search.function.DocValues;

import java.io.IOException;
import java.io.Serializable;

/**
 * Expert: source of values for basic function queries.
 * <P>
 * At its default/simplest form, values - one per doc - are used as the score of
 * that doc.
 * <P>
 * Values are instantiated as
 * {@link org.apache.lucene.search.function.DocValues DocValues} for a
 * particular reader.
 * <P>
 * ValueSource implementations differ in RAM requirements: it would always be a
 * factor of the number of documents, but for each document the number of bytes
 * can be 1, 2, 4, or 8.
 * 
 * <p>
 * <font color="#FF0000"> WARNING: The status of the <b>search.function</b>
 * package is experimental. The APIs introduced here might change in the future
 * and will not be supported anymore in such a case.</font>
 * 
 * 
 */
public abstract class ValueSource implements Serializable {

	/**
	 * Return the DocValues used by the function query.
	 * 
	 * @param reader
	 *            the IndexReader used to read these values. If any caching is
	 *            involved, that caching would also be IndexReader based.
	 * @throws IOException
	 *             for any error.
	 */
	public abstract DocValues getValues(IndexReader reader) throws IOException;

	/**
	 * description of field, used in explain()
	 */
	public abstract String description();

	/*
	 * (non-Javadoc)
	 * 
	 * @see java.lang.Object#toString()
	 */
	public String toString() {
		return description();
	}

	/**
	 * Needed for possible caching of query results - used by
	 * {@link ValueSourceQuery#equals(Object)}.
	 * 
	 * @see Object#equals(Object)
	 */
	public abstract boolean equals(Object o);

	/**
	 * Needed for possible caching of query results - used by
	 * {@link ValueSourceQuery#hashCode()}.
	 * 
	 * @see Object#hashCode()
	 */
	public abstract int hashCode();

}
